Acute graft-versus-host disease (GVHD) is a major obstacle to successful cancer therapy using allogeneic stem cell transplantation. Although it has long been known that GVHD is caused by T cells in the donor marrow that home to and injure critical target organs (skin and squamous mucosae, liver, and gut), the pathogenesis of GVHD has remained obscure. This is not surprising when one considers that the precise identity of a) subpopulations of donor T cells that are allostimulated, b) the specific target cells within the organs that are injured, and c) the very mechanism(s) of this injury have until recently been obscure. Using a relevant murine model of experimental GVHD, families of allostimulated effector T cells, identifiable via their Vp T cell receptor usage, now have been identified as specific effectors of epithelial target cell injury. Moreover, the targeted cells reside at the tips of murine lingual rete ridge-like prominences where they distinctively express cytokeratin 15 (RLPK15 cells) and where they are intimately associated with antigen- presenting dendritic cells. The mechanism of the demise of these target cells was found to be apoptosis. Thus, it is now possible to identify and enrich those Vp effector T cell families that are specifically allostimulated and employ them to induce experimental GVHD where specific subpopulations of targets cells may now be identified, isolated, and studied. Using this approach, this proposal is focused on three fundamental questions germane to GVHD pathogenesis, and ultimately to translational strategies for therapy;namely: i) how are RLPK15 target cells, as compared to K15-negative basal cells, influenced during the evolution of various GVHD effector pathways;ii) what are the specific apoptotic pathways that injure RLPK15 target cells;and iii) how does peripheral costimulation influence apoptosis of RLPK15 cells? Aside from further defining the basic pathobiology of GVHD, the information we will learn could pave the way for novel therapeutic strategies whereby protective transgenes are introduced specifically into cytokeratin 15- expressing GVHD target cells